Conventionally, there are various printed matters that has an original image copying of which is inhibited, which include certificates, important documents, etc., such as a resident card or a copy of a family register, and negotiable securities, such as a bank note, a stock certificate, a debenture, etc. In order to prevent unauthorized copying of the original images of such printed matters which is not permitted, there are proposed several methods and devices for enabling recognition and distinction that the reproduced image is a copy of the original image at the time of reproduction of the image.
For example, Japanese Laid-Open Patent Application No. 2004-260341 discloses an image processing device which includes an image combining unit which superimposes and combines the target image and the image pattern which indicates that copying of the target image is inhibited, and a data output unit which outputs the image data obtained after the combination of the target image and the image pattern, to a predetermined output device.
Also, Japanese Laid-Open patent Application No. 2004-260341 discloses an image processing method for distinguishing a background part and a latent image part of an image. In this image processing method, a ground tint image generating unit generates a set of dots for the latent image part which is expressed with a first number of pixels, and a set of dots for the background part which is expressed with a second number of pixels which number is different from the first number of pixels. A new image is generated in this manner. An additional information unit in the image processing method controls each position of the dot group contained in the latent image part of the image according to the bit value of the bit position corresponding to the additional information.
In addition, the sheet called the forged prevention sheet with which a special printing is performed beforehand is used. On this sheet, the mark (for example, characters, such as “copy inhibition”) which indicates that the image of the sheet is a copy of the original image is printed with an optical density that can be reproduced by a copier. On the other hand, the background part of the image is printed with an optical density that is difficult to be reproduced by the copier. The optical densities of the mark part and the background part are provided such that it is difficult for the human eyes to distinguish the mark part and the background part. The mark part and the background part formed in the areas other than the document part of the image are called a “ground tint”.
It is demanded that a ground tint is embedded in a document image without spoiling the information on the document as much as possible, and that a ground tint pattern is a pattern which can be distinguished at the time of copying that it is a copy of the original image. And printing of a ground-tint pattern is already realized by the electrophotographic printing method, such as that is used in a laser printer.
In recent years, not only the image forming device using the electrophotographic printing method, but also the image forming device (the ink-jet printing device) using the ink jet printing method in which the liquid ejection head is used as a printing head which carries out ejection of the recording liquid (ink) has been spread.
It is known that the ink-jet printing device uses as the printing head the liquid ejection head. The liquid ejection head comprises a pressure generating unit which generates the pressure which pressurizes the ink of the liquid chamber which communicates with the nozzle which ejects the drop of the ink. The pressure is applied to the ink of the liquid chamber by driving the pressure generating unit of the liquid ejection head in accordance with the image information. A droplet of the ink is ejected from the nozzle, and it is made to adhere to a recording medium, such as a sheet or a film, so that an image is formed on the recording medium (printing).
The dot position and the dot size that which can be formed, and the dot spreading on the recording medium are different between the printing of the ink jet printing method and the printing of the electrophotographic printing method. That is, according to the electrophotographic printing method, the dots can be formed at substantially arbitrary positions. However, according to the ink jet printing method, the dot position that can be formed by the printing head is restricted by the nozzle pitch and the drive frequency of the printing head.
Moreover, according to the electrophotographic printing method, the toner transferred to the recording medium does not permeate the recording medium. However, according to the ink jet printing method, the printing liquid does permeate the recording medium after the ink is ejected to the recording medium, and the dot spreading arises.
Therefore, when performing the printing using the ink-jet printing device which ejects a droplet of the recording liquid and forms an image (printing), there is a problem as to how to perform the printing of an image pattern which indicates that copying of a document image is inhibited.